Arb Notes for Feb 7th - Southern Minnesota’s Karst Country

February 12, 2013 at 9:57 am

By Callum McCulloch '15

Karst diagram courtesy of Vancouver Island University.

Although the Cowling Arboretum does not exhibit any karst topography, much of Southern Minnesota does. Karst is a geological feature formed by the dissolution of soluble bedrock such as carbonates like limestone and dolostone. Karst formations lead to the formations of caves, disappearing streams, underground streams, sinkholes and other landforms in Southern Minnesota. The longest cave system in the world, the Mammoth-Flint Ridge Cave System in Kentucky was formed through the dissolution of carbonate rocks in its karst area.

Karst country in Southern Minnesota coincides with the Driftless Area that covers Southeastern Minnesota, Northwestern Iowa and Western Wisconsin. Glacial drift no younger than 500,000 years old has been discovered in Southern Minnesota Driftless Area, meaning it has not been glaciated in that time. Other geologists believe that the Driftless Area has not been glaciated in at least 2 million years. However, the Driftless Area has been subject to glacial lake outburst floods when titanic lakes like proglacial Lake Duluth began to cataclysmically drain about 9,500 years ago.

Karst forms when slightly acidic water meets a weakly soluble carbonate rock. Rainwater acidifies ever so slightly as it passes through the atmosphere and takes up CO2. As rainwater travels through the soil it picks up more CO2 and forms a weak carbonic acid solution, which readily dissolves carbonate rocks over time. Limestone is removed from the site in the form of calcium and bicarbonate.

A good way to spot a karst sinkhole in Southern Minnesota is to look for a tree covered area in the middle of a farmer’s field that she is wise not to plow.